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Carbon nanotubes


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Carbon nanotubes

  2. 2. ACKNOWLEDGEMENT  I would like to express my special thanks of gratitude to my teacher MISS Sonalika as well as our H.O.D Mr.Prabhat jain sir_who gave me the golden opportunity to do this wonderful project on the topic “ CARBON NANOTUBES “ which also helped me in doing a lot of Research and i came to know about so many new things. I am really thankful to them. Secondly I would also like to thank my friends who helped me a lot in finishing this project within the limited time. I am making this project not only for marks but to also increase my knowledge . THANKS AGAIN TO ALL WHO HELPED ME
  3. 3. introduction Carbon nanotubes (CNTs) are allotropes of carbon. These cylindrical carbon molecules have interesting properties that make them potentially useful in many applications in nanotechnology, electronics, optics and other fields of materials science, as well as potential uses in architectural fields. They exhibit extraordinary strength and unique electrical properties, and are efficient conductors of heat.
  4. 4. History In 1985, a molecule called buckminsterfullerene was discovered by a group of researchers at Rice University. This molecule consisted of 60 carbon atoms in sp 2 hybridized bonds arranged in a surprisingly symmetric fashion. The Nobel Prize was awarded to Richard Smalley, Robert Curl, and Harry Kroto for their discovery of this new allotrope of carbon. This discovery was groundbreaking for the now vibrant field of carbon nanotechnology. Carbon nanotubes, discovered in 1991 by Sumio Iijima, are members of the fullerene family. Their morphology is considered equivalent to a graphene sheet rolled into a seamless tube capped on both ends
  5. 5. Chemistry of CNT Carbon is the most versatile element in the periodic table, owing to the type, strength, and number of bonds it can form with many different elements. The diversity of bonds and their corresponding geometries enable the existence of structural isomers, geometric isomers, and enantiomers. Graphite ( Ambient conditions) sp2 hybridization : planar Diamond (High temperature and pressure) sp3 hybridization: tetrahedral Nanotube/Fullerene (certain growth conditions) sp2 + sp3 character: cylindrical
  6. 6. Finite size of graphene layer has dangling bonds. These dangling bonds correspond to high energy states. Eliminates dangling bonds Increases Strain Energy Total Energy decreases + Nanotube formation
  7. 7. CNT are rolled graphene sheets
  8. 8. Types of CNTs  Single Wall CNT (SWCNT)  Multiple Wall CNT (MWCNT)
  9. 9. SWCNT  a single-walled carbon nanotube (SWCNT) is a structure rolled from a sheet of graphene. It can be divided into three kinds, such as armchair, zigzag and chiral type, according to its structure. However, it can also be divided into three categories, including metal, narrow-gap semiconductor and moderate-gap semiconductor for its electronic property, according to its rolling direction. The helix of a SWNT causes the six rings of a SWNT’s wall distorted, and makes unsaturated double-bonds on the rings of a SWNT to provide resistance while electrons being conducted. Hence, differences in the electronic property of a SWNT occur
  10. 10. Armchair (n,m) = (5,5) = 30 Chiral (n,m) = (10,5) 0 < < 30 The graphene sheet labeled with the integers (n, m). The diameter, chiral angle, and type can be determined by knowing the integers (n, m). The chiral angle is used to seprate carbon nanotubes in three classes differentiated by their electronic properties: armchair (n = m, θ= 30 ˚), zig-zag (m = 0, n > 0, θ = 0 ˚), and chiral (0 < |m| < n, 0 < θ < 30 ˚)
  11. 11. MWCNT Multi-wall nanotubes can appear either in the form of a coaxial assembly of SWNT similar to a coaxial cable, or as a single sheet of graphite rolled into the shape of a scroll. The diameters of MWNT are typically in the range of 5 nm to 50 nm. The interlayer distance in MWNT is close to the distance between graphene layers in graphite. MWNT are easier to produce in high volume quantities than SWNT. However, the structure of MWNT is less well understood because of its greater complexity and variety. Regions of structural imperfection may diminish its desirable material properties.
  12. 12. METHODS FOR PREPRATION OF CNTs Arc-Evaporation Arc-evaporation synthesis, also known as electric arc discharge, has long been known as the best method for synthesizing fullerenes, and it also generates the highest quality carbon nanotubes. Arc-evaporation apparatus consists of two graphite electrodes under helium. A current of around 50A is passed between the electrodes, causing some of the graphite from the anode to evaporate and condense on the cathode - this deposit contains the carbon nanotubes. Arc-evaporation with pure graphite electrodes produces mainly multi-wall nanotubes, although single-wall nanotubes can be made by this method by doping the anode with a metal catalyst such as cobalt or nickel. Whilst the nanotubes produced by arc discharge are of a very high quality, they are mixed with a large amount of amorphous carbon, which makes this technique difficult to scale up.
  13. 13. Laser Vaporization The laser vaporization (or laser ablation) method was developed in 1995. A pulsed laser is fired at a graphite target in an inert environment, at high temperature and pressure. The target is usually placed at one end of a 50cm quartz tube - the nanotubes are collected at the opposite end. The shape and structure of the nanotubes produced by laser vaporization are more easily controllable, as they are only affected by a small number of parameters. The yield of carbon nanotubes is also much higher, with very little amorphous carbon produced, but the overall amount generated is very small. This combined with the high operating temperature and pressure make this method highly inefficient for producing large amounts of carbon nanotubes.
  14. 14. Chemical Vapour Deposition (CVD) Chemical vapour deposition is the method with the most promise for mass production of carbon nanotubes. It operates at much lower temperatures, and produces nanotubes in greater quantities than arc discharge or laser vaporization. CVD uses a carbon-rich gas feedstock, such as acetylene or ethylene (IUPAC ethyne, ethene). The gas is passed over a metal nanoparticle catalyst (typically iron, nickel, or molybdenum) which has been deposited on a porous substrate (e.g. silica, alumina). Carbon atoms dissociate from the gas molecules as they pass over the catalyst, rearranging on the surface to form nanotubes and fullerenes. This allows nanotubes to be synthesized continuously, making the technique ideal for scaling up to large manufacturing volumes.
  15. 15. Other Synthetic Methods Several other methods for producing carbon nanotubes have been reported, which are mostly at an early stage of research. Some of these may have the potential to be good mass-production methods, with further development. •Diffusion flame synthesis, with a variety of metal and metal oxide catalysts and support geometries •Electrolysis of graphite in molten lithium chloride under an inert atmosphere •Ball milling and annealing of graphite, catalyzed by iron contamination from the steel milling balls •Heat treatment of polyesters formed from citric acid and ethylene glycol, at 400C in air •Hydrothermal treatment of polyethylene with a nickel catalyst under high pressure •Explosive decomposition of picric acid, in the presence of cobalt acetate and paraffin, produces a high yield of relatively homogeneous, "bamboo-shaped" carbon nanotubes.
  16. 16. Electrical Properties If the nanotube structure is armchair then the electrical properties are metallic If the nanotube structure is chiral then the electrical properties can be either semiconducting with a very small band gap, otherwise the nanotube is a moderate semiconductor In theory, metallic nanotubes can carry an electrical current density of 4×109 A/cm2 which is more than 1,000 times greater than metals such as copper
  17. 17. Thermal Properties All nanotubes are expected to be very good thermal conductors along the tube, but good insulators laterally to the tube axis. It is predicted that carbon nanotubes will be able to transmit up to 6000 watts per meter per Kelvin at room temperature; compare this to copper, a metal well-known for its good thermal conductivity, which transmits 385 watts per meter per K. The temperature stability of carbon nanotubes is estimated to be up to 2800 C in vacuum and about 750 C in air.
  18. 18. Defects Defects can occur in the form of atomic vacancies. High levels of such defects can lower the tensile strength by up to 85%. Because of the very small structure of CNTs, the tensile strength of the tube is dependent on its weakest segment in a similar manner to a chain, where the strength of the weakest link becomes the maximum strength of the chain
  19. 19. applications Carbon nanotubes, miniscule pipes of rolled up carbon atoms, have amazing properties that have taken the world by storm. They are currently being integrated into hundreds of different applications, from green tech to clothing and medicine. Here’s a peak at some uses for this wonder material. Its size, surface area (500 square meter per gram), and adsorption properties make carbon nanotubes an ideal membrane for filtering toxic chemicals, dissolved salts and biological contaminants from water.
  20. 20. Carbon nanotubes have been added to strengthen materials for sports equipment, body armor, vehicles, rockets, and building materials. Using carbon nanotubes as the electrodes in capacitors provides more current and better electrical and mechanical stability than other leading materials Carbon nanotubes are perfect for allowing damaged bone to restructure itself: they’re strong, lightweight, and can be modified for compatibility with any part of the body.
  21. 21. From flat screens, to LEDs to flexible displays, nanotubes will increase your viewing pleasure and portability. These tiny pipes of carbon make excellent field emitters or conductive surfaces. They’re strong, they’re elastic, and they have amazing electrical properties. Researchers have created a carbon nanotube aerogel that expands and contracts as it converts electricity into chemical energy.
  22. 22. Health Hazards According to scientists at the National Institute of Standards and Technology, carbon nanotubes shorter than about 200 nanometers readily enter into human lung cells similar to the way asbestos does, and may pose an increased risk to health. Carbon nanotubes along with the majority of nanotechnology, are an unexplored matter, and many of the possible health hazards are still unknown.